Composite parts and structures are finding ever wider use because of their high strength to weight ratio. Some composites are attractive because they also resist high temperatures. Gas turbine exhaust liners are one environment where these qualities are especially attractive. Ceramic components are especially attractive because they resist the extremely high gas turbine exhaust temperatures, although they do have significantly less strength than pure metal components. Ceramic composites such as Nextel 440/Silica display a rather brittle matrix structure, one that tends to powder when exposed to impact and point loads. That characteristic presents challenges when mounting ceramic matrix components, such as liners in an engine exhaust. A conventional bolt can create extremely high point loads, fracturing the ceramic surface. In other words, the surface can be damaged simply from the compression if a conventional screw bolt is used to fasten the ceramic in place. In addition, vibration produces mechanical loads that generate impact and point loads on the ceramic's surface. Similar loads, created from large differences in thermal expansion, can damage the ceramic. For instance, thermal radial growth by a bolt in a hole or bore in a composite structure can exert substantial forces, producing either or both a crack and bearing failure.
Some of these problems have been considered in other applications in the prior art. For instance, elastomeric bushings are used to reduce noise and vibration, but they are not resistant to high temperatures and do not provide firm support. Washers are used to distribute compression loading, but they can distort, creating high point loads. U.S. Pat. No. 4,834,569 shows a technique in which an insert or bushing is placed in a composite core. U.S. Pat. No. 4,490,083 shows a technique that uses a deformable insert to protect the interface between a fastener and the bore walls. U.S. Pat. No. 4,790,683 shows a method that employs a tolerance ring in conjunction with a relatively soft material to prevent damage (deformation) to the material. However, none of these techniques make a significant contribution towards protecting brittle ceramic composites from all of the forces created by a fastener placed in a high temperature, high vibration environment, such as if found in the exhaust of a gas turbine engine.